NFC technology allows for simplified wireless data exchanges between two NFC-enabled devices over a range of several centimeters or less. For example, an NFC-enabled mobile phone or a smart card having an NFC/RFID tag may exchange data with an NFC reader (e.g., in a point-of-sale terminal or another mobile device), thereby allowing a customer to purchase goods or services without exchanging hard currency or physically swiping a credit card. NFC technology may also be used to facilitate social networking, contact sharing, and/or establishing other wireless connections (e.g., Bluetooth or WiFi).
To establish an NFC connection between an initiator device and a target device, both devices follow a number of NFC standards. Examples of such NFC standards include ISO/IEC 18092 and ECMA-340 standards, which define modulation schemes, encoding and decoding schemes, transfer rates, frame format, transmission protocols, and so forth, for an NFC connection. More specifically, to initiate an NFC connection with the target device, the initiator device transmits an un-modulated radio frequency (RF) carrier signal for an active RF guard time (currently set to approximately 5 ms), and then modulates the carrier signal to embed a polling command (e.g., request frame) that requests either an active communication mode or a passive communication mode. If the initiator device requests the active communication mode, the initiator device terminates transmission of its carrier signal after transmitting the polling command, and then the target device transmits data to the initiator device by generating and modulating its own RF carrier signal. Conversely, if the initiator device requests the passive communication mode, the initiator device continues transmitting its carrier signal, and the target device transmits data to the initiator device by load modulating the initiator device's carrier signal. Thus, for the active communication mode, power consumption is shared between the initiator device and the target device, while for the passive communication mode, the target device consumes very little (if any) power because it does not generate its own carrier signal.
More specifically, when the initiator device requests the active communication mode in its polling command, the target device is to generate and transmit its own carrier signal no later than a predetermined response time after the initiator device terminates its carrier signal transmission. Currently, the predetermined response time for active mode communications, which is sometimes referred to as the active delay time (TADT), is set at 302 μs by the ISO 18092 standards. As mentioned above, the initiator device typically terminates its carrier signal immediately after transmitting the polling command to the target device, for example, so that the initiator device can receive data transmitted from the target device via the target device's own carrier signal. Because many clock generators take much longer than 302 μs to generate and stabilize a clock signal suitable for generating and modulating an NFC carrier signal, the target device typically enables its clock generator immediately after detecting the initial un-modulated carrier signal transmitted from the initiator device and/or generates its own carrier signal only after determining that the initiator device has terminated its carrier signal. In this manner, the target device may have sufficient time to enable its clock generator, stabilize its clock signal, and transmit its own carrier signal within the active delay time (TADT). However, if the target device enables its clock generator prematurely or unnecessarily, power consumption may be unnecessarily consumed. For example, if the initiator device subsequently requests the passive communication mode (e.g., after transmitting its un-modulated carrier signal for the active RF guard time (5 ms)), then the target device does not need to generate and transmit its own carrier signal). This unnecessary power consumption is of particular concern when the target device is a mobile device having a limited power supply (e.g., a smartphone powered by a small battery.
Accordingly, there is a need to reduce power consumption associated with establishing an NFC connection between NFC-enabled devices.